Strategy

Ancient Walls are the first defense available in the game, good against the first regular weapons carried by the likes of the Warrior and the Spearman. However, siege equipment quickly makes them obsolete.

The new rules of city combat make Walls much more important than in Civilization V. Try to build them as soon as possible to greatly fortify your city against potential destruction, especially if you are facing an early war or invasion threat. As Outer Defense strength ensures the Ranged strike ability, and reduces greatly damage from non-siege weapons, it is vital to ensure that all of your cities have built some Walls. As you progress further into the game, Ancient Walls are required to build Medieval Walls and beyond.

Civilopedia entry

Until things like gunpowder, bomber aircraft, and nuclear missiles made them rather pointless, city walls were pretty important if a city wanted to stay inviolate. If a town wanted to keep its citizens relatively safe and its stuff relatively secure from the barbarians at the gates, it needed a relatively strong wall surrounding it. The Sumerian city of Uruk is the oldest known settlement completely enclosed by a wall, in this case supposedly built by the god-king Gilgamesh. Some of the early settlements in the Indus Valley – such as Kot Diji – had massive stone-and-brick walls for protection. On the other hand, if the city was inhabited by awesome warriors, it could generally get by without walls – Sparta and Rome survived for several centuries before walls were built. Today, what portions of city walls remain are either ignored or consider part of the “local color.”